Optical encoders are typically employed as motion detectors in applications such as closed-loop feedback control in motor control systems. By way of example, many optical encoders are configured to translate rotary motion or linear motion into a two-channel digital output for position encoding.
Many optical encoders employ an LED as a light source. In transmissive encoders, the light is collimated in to a parallel beam by means of a lens located over the LED. Opposite the emitter is a light detector that typically consists of photo-diode arrays and a signal processor. When a code scale such as a code wheel or code strip moves between the light emitter and light detector, the light beam is interrupted by a pattern of bars and spaces disposed on the code scale. Similarly, in reflective or imaging encoders, the lens over an LED focuses light onto the code scale. Light is either reflected or not reflected back to the lens disposed over the photo-detector. As the code scale moves, an alternating pattern of light and dark patterns corresponding to the bars and spaces falls upon the photodiodes. The photodiodes detect these patterns and corresponding outputs are processed by the signal processor to produce digital waveforms. Such encoder outputs are used to provide information about position, velocity and acceleration of a motor, by way of example.
A typical reflective optical encoder comprises a light detector, a light emitter and a code wheel or code scale. The detector generates an output by processing photo currents provided by photodiode arrays included in the light detector. In general, reflective optical encoders include four photodiode channels, namely A, A\, B and B\, which are arranged along a single track in a 2-channel optical encoder. The photodiodes are arranged so that gaps separating adjacent photodiodes are sufficiently large to prevent or inhibit crosstalk from being generated between such adjoining photodiodes. In the prior art, as the resolution of an optical encoder increased, the spacing between adjoining photodiodes decreased, which in turn led to increased crosstalk between channels.
FIG. 1 shows a conventional prior art single track optical encoder 10 with photodiode array 20 comprising detectors A, A\, B and B\ in a two-channel encoder with associated code strip 30. Signals generated by detectors A and A\ (channel A) and B and B\ (channel B) are also shown in FIG. 1, where the Channel B output signal lags the Channel A output signal by 90 degrees. (The relatively simple circuitry employed to generate output signals for channels A and B is not shown in FIG. 1, but is well known to those skilled in the art and therefore need not be discussed further herein.) The separation between adjoining photodiodes in array 20 and the width of each photodiode is selected according to the resolution that is required of the optical encoder. When the resolution of optical encoder 10 is increased, either the spacing w between adjoining photodiodes is reduced, or the width of each photodiode along common axis 15 is reduced, or both, resulting in photodiode spacing z being decreased, where z is the spacing between the leading or trailing edges of adjoining photodiodes. Continuing to refer to FIG. 1, note that within a distance X″/2 of photodiode array 20 two photodiodes are to be found.
FIG. 2 shows another conventional prior art single track optical encoder 10 with photodiode array 20 comprising detectors A, A\, B and B\ in a two-channel encoder with associated code strip 30. Optical encoder 10 of FIG. 2 has twice the spatial resolution of encoder 10 of FIG. 1: note that code scale 30 shown in FIG. 2 has alternating bands of substantially reflective portions 31 and substantially non-reflective portions 33 having a combined length of X″″/2, which is half that of FIG. 1 (X″). As illustrated in FIG. 2, the minimum separation between adjoining photodiodes or photodetectors A, B, A\ and B\ is also w. Crosstalk between adjoining photodiodes is generated between the 2 channels of encoder 10 if the separation between adjoining photodiodes is less than w. When the resolution of photodiodes A, B, A\ and B\ is increased (i.e., the widths of such photodiodes are decreased), spacing Z is reduced. Hence, either the widths of the individual photodiodes within spacing z or the separation between the photodiodes w needs to be reduced. Spacing z and inter-photodiode separation w limited by the process technology which has been selected (e.g., CMOS, BiCMOS, etc.).
FIG. 3 shows another conventional prior art single track optical encoder 10 with photodiode array 20 comprising detectors A, A\, B and B\ in a two-channel encoder with associated code strip 30. Optical encoder 10 of FIG. 3 has four times the spatial resolution of encoder 10 of FIG. 1: note that code scale 30 shown in FIG. 2 has alternating bands of substantially reflective portions 31 and substantially non-reflective portions 33 having a combined length of X″″/4, which is one-quarter that of FIG. 1 (X″). As illustrated in FIG. 3, the minimum separation between adjoining photodiodes or photodetectors A, B, A\ and B\ is less than w, and as a result crosstalk between adjoining photodiodes will be generated. Photodiode width y shown in FIG. 3 is the minimum width of a photodiode for a selected manufacturing process (e.g., CMOS, BiCMOS, etc.). Crosstalk thus occurs between the A and B channels, as the separation w between adjoining photodiodes violated the photodiode separation rule. Moreover, the small width y of each photodiode results in a small amount of electrical current being generated by each photodiode, as the current generated by each photodiode is proportional to the amount of each photodiode's surface area. Hence, the performance of the encoder is affected because cross-talk is generated while the signal-to-noise ratio is low. As a result, and to achieve sufficiently high resolution using a conventional optical encoder, additional circuitry (such as interpolation, filtering or amplification circuitry) may be required to provide adequate performance. Such additional circuitry, of course, increases the cost and size of the encoder.
Note that in each of optical encoders 10 illustrated in FIGS. 1, 2 and 3 the spacing z between the leading or trailing edges of adjoining photodiodes corresponds to one-quarter the combined width of a single pair of adjoining light and dark strips on code scale 30. As a result, two photodiodes are contained within a distance defining the length of each such strip along common axis 15. Note further that in each of optical encoders 10 illustrated in FIGS. 1, 2 and 3 all photodiodes disposed along single track or common axis 15 are arranged in the order or sequence A, B, A\, and B\.
The market demands ever smaller and higher resolution optical reflective encoders. What is needed is a smaller, higher resolution optical reflective encoder that can be provided without the use of complicated, expensive, signal processing output circuitry.
Various patents containing subject matter relating directly or indirectly to the field of the present invention include, but are not limited to, the following:
U.S. Pat. No. 5,148,020 to Machida, Sep. 15, 1992;
U.S. Pat. No. 6,727,493 to Franklin et al., Apr. 27, 2004;
U.S. Pat. No. 7,145,128 to Tanaka, Dec. 5, 2006;
U.S. Pat. No. 7,276,687 to Okada et al., Oct. 2, 2007, and
U.S. Pat. No. 7,449,675 to Chong et al., Nov. 11, 2008.
The dates of the foregoing publications may correspond to any one of priority dates, filing dates, publication dates and issue dates. Listing of the above patents and patent applications in this background section is not, and shall not be construed as, an admission by the applicants or their counsel that one or more publications from the above list constitutes prior art in respect of the applicant's various inventions. All printed publications and patents referenced herein are hereby incorporated by referenced herein, each in its respective entirety.
Upon having read and understood the Summary, Detailed Description and Claims set forth below, those skilled in the art will appreciate that at least some of the systems, devices, components and methods disclosed in the printed publications listed herein may be modified advantageously in accordance with the teachings of the various embodiments of the present invention.